1. Field of the Invention
The present invention relates to a silica glass, optical members including the glass, and a method for producing the glass More particularly, it relates to a silica glass used for optical systems which use ultraviolet light of 400 nm or shorter and light of vacuum ultraviolet wavelength region, such as excimer laser lithography, optical members and a method for producing the silica glass.
2. Related Background Art
Hitherto, exposure apparatuss called stepper are used in photolithographic techniques according to which fine patterns of integrated circuits are exposed and transferred onto wafers such as of silicon. With recent increase in integration of LSI, wavelength of light sources in the stepper becomes gradually shorter such as g-line (436 nm), i-line (365 nm), KrF excimer laser beam (248 nm) and ArF excimer laser beam (193 nm). In general, synthetic silica glasses are used as optical glasses for lenses of illumination optical systems or projection optical systems of steppers because high transmittance for light of wavelength region shorter than i-line and ultraviolet light resistance are required.
However, even in the case of the synthetic silica glasses, when they are irradiated with ultraviolet light of high output or excimer laser beam for a long time, there appear an absorption region of 215 nm produced by structural defect called E' center (having a structure of .tbd.Si where .tbd. does not mean a triple bond, but means that Si bonds to three oxygen atoms and "." indicates an unpaired electron) and an absorption region of 260 nm produced by structural defect called NBOHC (Non-Bridging Oxygen Hole Center; having a structure of .tbd.Si--O.) which cause considerable decrease of transmittance for light of ultraviolet region. These silica glasses inferior in ultraviolet light resistance are not suitable as optical members such as optical lenses for ultraviolet light mentioned above.
For improving the ultraviolet light resistance of such silica glasses, it is known to subject silica glasses to a heat treatment in a hydrogen atmosphere (e.g., Japanese Patent Application Laid-Open Gazette (Kokai) No. 1-201664). Furthermore, it is proposed to improve excimer resistance by doping silica glasses with hydrogen molecules (e.g., Japanese Patent Application Laid-Open Gazette (Kokai) No. 3-109233).